Electrically heated salt bath furnace



June 23, 1931. A. G. E. HULTGREN 1,811

EEECTRICALLY HEATED SALT BATH FURNACE Filed June 10, 1930 Patented June23, 1931 UNITED STATES PATENT OFFICE MIGALLY mm SALT BATH FURNACEApplication am June 10. mo. Serial 80. 400,231, and in Sweden Februaryas, mo.

The present invention refers to electrically heated salt bath furnaces,to those emplofled for hard of highsteel too Two common esnso sufurnaces I areshowninFig. land 2. Therefractory crucible 1 contains emolten salt 2. Heat is supplied by alternating current passing betweenthe electrodes 3 through the salt. The tools are heated b immersion inthe salt,

10 which is held at the esired temperature by regulating the current.Owing to convection currents, the temperature of the salt becomes veryuniform. This, normally, applies to the temperature of the tool also,prol vided the time of immersion is suflicient. By measuring thetemperature of the salt, the temperature of the tool, with the exceptionreferred to below, is accurately determined. If the compositions of. thesalt and the linlO ing are suitably chosen, oxidatlon of, or otherchemical reaction with, the surface of the tool is practicallyprevented. This is not the case with certain other methods for heatingof high-speed steel.

' In the arrangement shown in Fig. 1, the

electrodes are usually so connected to the current supply that thecurrent passes between adjacent, not between opposite electrodes. InFig. 2 three-phase current is used, the

current passing between each electrode and the two others. During theuse of such furnaces, however, particularly when large objects wereheated, signs of local or general overheating of the tool have oftenoccurred, usually in the form of partial meltin of the steel. This hasbeen so, in spite of t e temperature of the salt, as measured bypyrometer, having been correct and lower than the a melting point of thesteel.

is that, owing to the higher conductivity of the steel as compared withthat of the salt, and because of the distance between the tool and thecorresponding electrodes having been I too small in comparison to thedistance between the electrodes, a considerable part of the current haspassed through the tool thus overheating it. The portions of the toolfacing the electrodes are particularly liable to II overheating andmelting phenomena. In

The explanation this way expensive tools have sometimes been destroyed.

In order to compensate for overheating from such cause, the salt may bekept at a correspondin ly lower temperature, but it is obvious that ythis method uncertain and uneven tem ratures in the tool, sometimes toolow an sometimes too high, will be attained.

According to the present invention, the current is passed practicallyentirely through the salt, not, or only a negligible part of it, throughthe immersed tool, even if the latter is re atively large, thus anundesirable and detrimental overheating o the steel being prevented.This is accomplished by a more suitable arrangement of the electrodes,preferably in connection with a more suitable shape of the latter and ofthe crucible.

Various designs of the invention are shown in Figs. 3 to 10, Figs. 3 to5 adapted for one-p se current, a single circuit, Figs. 6 to 8 forone-phase current, two circuits, and Figs. 9 and 10 for three-phasecurrent. As is seen, the electrodes are always arranged in pairs. Inhorizontal section, the inner faces of co-ordinated electrodes areparallel and comggratively close to each other, the distance in measuredin the direction of the current. experiment, it has been determined thatthat distance should nowhere exceed 2 of the horizontal dimension of thesalt bat in the same direction. Coordinated electrodes are preferablymade to approach each other somewhat towards the bottom of the crucible.This feature, however, is not shown in the figures, which are schematic.

Experiments with the construction indicated in Figs. 3 to 10 have shownthat there is no danger of overheating the immersed tool above' thetemperature of the salt, if

the tool is kept outside of a zone surround ing the electrodes, thewidth of which is agproximatel equal to the distance between t e latter.though, according to the .invention only a small part of the salt isdirectly heated b the assing current, the temperature of t e who e bathbecomes practlcally uniform, owing to the convection currents.

the objects to be heated, and the number of the objects to be heatedsimultaneously. After room has been provided for the purpose, one ormore pairs of electrodes of appropriate size are added in the mannerdescribed above, and at a distance from the contour of said room equaltoor somewhat exceeding the distance between two co-ordinated electrodes.The number of pairs of electrodes is determined by the type of currentto be employed. Around the whole a figure is then circumscribed whichforms the inner contour of the crucible.

As has been stated above, the inner faces of coordinated electrodes inhorizontal sector, are preferably parallel, but deviations from thiscondition may be made without departing from the main principle of theinvention.

In all the figures, straight lines have been drawn between co-ordinatedelectrodes in order to show schematically the paths of the currentthrough the salt. On comparing Fig. 1 with Figs. 68 and Fig. 2 withFigs. 9-10, it is seen that, in the old designs of salt bath furnaces,the part of the salt available forheating of tools without danger ofoverheating b current passing through them, is considera 1y smaller thanin the design ac cording to present invention.

In Figure 11 there is shown one form of my invention, wherein theelectrodes of each pair approach each other towards the bottom of thefurnace.

The present construction has been developed particularly for salt bathfurnaces for hardening of high-speed steel but may also be applied toother salt bath furnaces. for heating of metal objects, in which heat issupplied by electric current passing through the salt betweenelectrodes.

Having now described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. Salt bath furnace for heating of tools and other metal objects, inwhich heat is supelectrodes, each pair of the said co-ordinatedelectrodes being so disposed that in an horizontal section the distancebetween a j acent faces is constant, or approximately constant, alongtheir whole width.

3. Salt bath furnace for heating of tools and other metal objects, inwhich heat is supplied by electric current assing throu h the salt,having an even num er of electro es arranged in pairs in the crucible insuch a manner that the space to be occupied by the immersed objects isnot traversed by any straight current paths drawn between coordinatedelectrodes, the horizontal distance between the said co-ordina-tedelectrodes in the direction of the current path nowhere exceeding halfof the maximum internal width of the crucible in the same direction.

4. Salt bath furnace for heating of tools and other metal objects, inwhich heat is supplied by electric-current assing through the salt,having an even num er of electrodes arranged in pairs in the crucible insuch a manner that the space to be occupied by the immersed objects isnot traversed by any straight current paths drawn between coordinatedelectrodes, each pair of electrodes approaching each other towards thebottom of the furnace.

In witness whereof I have hereunto signed my name.

AXEL GUSTAF EMANUEL HULTGREN.

' plied by electric current passing throu h the salt, having an evennumber of electro es arranged in pairs in the crucible in such a mannerthat the space to be occupied by the immersed objects is not traversedby any straight current paths drawn between coordinated electrodes.

2. Salt bath furnace for heating of tools and other metal objects, inwhich heat is supplied by electric current assing throu h the salt,having an even num er of electro es arranged in pairs in the crucible insuch a manner that the space to be occupied by the immersed objects isnot traversed by any straight current paths drawn between co-ordinatedIII

